The present invention relates to image pickup apparatus using an XY addressing type solid-state image pickup device.
Among conventionally known as XY addressing type image pickup device is one constructed as shown in FIG. 1. Referring to FIG. 1: numeral 1 denotes light receiving pixels; 2, optically shielded OB (optical black) pixels disposed on the periphery of the light receiving pixels; and 3, 4, vertical and horizontal scanning circuits, respectively, for scanning and reading out the light receiving pixels 1. Thus constructed XY addressing type solid-state image pickup device is known to be capable of selectively reading out signal of given pixels, i.e., random access, by suitably controlling the driving methods of the horizontal scanning circuit and the vertical scanning circuit.
Further, usually in a solid-state image pickup device, an optically shielded OB pixel is read out in a vertical or horizontal blanking period to perform OB clamping so as to bring its output level to a reference level in order to perform a stable image pickup operation against a change in ambient conditions such as in temperatures.
For example, driving methods are disclosed in Japanese patent application laid open No. 9-163236 of an image pickup apparatus using XY addressing type solid-state image pickup device having optically shielded OB pixels on the periphery of light receiving pixels, which can be driven in a thinned-out readout mode for reading out one light receiving pixel out of every given number of pixels and in a mode of reading out all the pixels, where: (1) OB pixels, too, are thinned out and read out at a low rate in the thinned-out readout mode for reading out one light receiving pixel out of every given number of pixels; (2) OB pixels are read out without being thinned in the thinned-out readout mode for reading out one light receiving pixel out of every given number of pixels; and (3) OB pixels are read out without being thinned out in the mode of reading out all the pixels.
Further, Japanese patent application laid open No. 9163244 discloses an image pickup device having optically shielded OB pixels on the periphery of light receiving pixels and adapted as capable of drive in a mode of reading out light receiving pixels from a given region, where OB pixels on the periphery of the light receiving pixels are read out in horizontal or vertical blanking period to thereby make a stable OB clamping possible even in the mode of reading out the light receiving pixels from a given region. Further, an XY addressing type solid-state image pickup device generally possesses a relatively large FPN (fixed pattern noise) and its suppression is required. Among the methods of suppressing such FPN, one is disclosed for example in Japanese patent application laid open No. 7-15666 where reset is effected after reading out video signals from pixels, and pixel signals read out immediately after such reset are written as FPN into a memory, suppressing FPN by subtracting the pixel signals thus written into the memory from the video signals.
As described above, an XY addressing type solid-state image pickup device is capable of random access by suitably adapting a scanning circuit for selecting pixels from which signals are read out. An image pickup device having horizontal by vertical pixels of Hxc3x97V=2048xc3x972048=4,200,000, for example, is capable, in addition to a normal full-pixel readout, of reading out only Hxc3x97V=512xc3x97512 pixels by thinning out to xc2xc both horizontally and vertically for the entire picture area or of reading out the light receiving pixels from a given pixel region, etc.
Here, a solid-state image pickup device where OB pixel 2-1 disposed as shown in FIG. 2A before light receiving pixels 1 of each line from which signals are read out is read out for OB clamping, when driven in a thinned-out readout mode for the entire picture area, a mode of reading out region xe2x80x9cAxe2x80x9d, and a mode of reading out region xe2x80x9cBBxe2x80x9d produces a displayed image on a monitor or the like as shown in FIGS. 2B, 2C, 2D, respectively.
In this respect, the manner of operation will now be described of the case where FPN suppressing operation is effected at the same time of OB clamping using OB pixels. The suppression of FPN is performed as follows. In particular, the solid-state image pickup device is driven so that signal immediately after reset is read out for the first one frame. Such signal is stored to a memory as FPN, and, for frames of the next and after, FPN thus stored at the memory is subtracted from video signal to suppress FPN. On the other hand, in view of stability, time constant of OB clamping is usually set to several-hundred ms. Accordingly, for example when a transition is made from the thinned-out readout mode to the mode of reading out region xe2x80x9cAxe2x80x9d, OB clamping during the time period for writing as FPN into the memory the signal immediately after reset in the first frame at such transition, is effected substantially in accordance with the output of OB pixels at the time of performing the thinned-out readout before the transition. It is gradually converged into OB clamping in accordance with the output of OB pixels corresponding to readout of region xe2x80x9cAxe2x80x9d.
For this reason, if shading occurs due to variance in vertical direction of the OB pixels to be read out for OB clamping, an output level of OB pixel to be read out differs from another depending on the location of readout of image in the thinned-out readout mode or in the mode of reading out region xe2x80x9cAxe2x80x9d. A difference thus occurs between DC level during the first one frame period for writing FPN into the memory and DC level after several-hundred ms when OB clamping has been stabilized. Accordingly, data to be written into the memory as FPN possesses an offset. Since FPN data possessing such offset is subtracted from video signal to suppress FPN, there is a problem that video output after FPN suppression, too, contains an error corresponding to the offset.
Moreover, OB clamping is necessary also when all the pixels in one complete shot are read out to take a picture. Since the time constant of an OB clamp circuit is usually several-hundred ms, a stable OB clamping cannot be performed due to the fact that one shot of picture is taken in a short time duration.
To eliminate the above problems in conventional image pickup apparatus having both OB clamp means and FPN suppression means, it is a main object of the present invention to provide an image pickup apparatus in which stable OB clamping is performed and FPN suppression can be correctly effected even when signal readout mode has been changed or when location and/or number of pixels to be read out have been changed.
In accordance with the present invention, there is provided an image pickup apparatus including: an XY addressing type solid-state image pickup device having light receiving pixels and optically shielded OB pixels on the periphery of the light receiving pixels; and drive means for driving the solid-state image pickup device in signal readout modes of both or one of a thinned-out mode for reading out one light receiving pixel out of every given number of pixels and a mode of reading out the light receiving pixels in a given region; and the apparatus further including means for effecting OB clamping by always reading out OB pixels at the same location, regardless of the two signal readout modes or regardless of location and/or number of light receiving pixels to be read out in the two signal readout modes.
Since OB clamping is effected by thus reading out always the OB pixels at the same location regardless of the two signal readout modes or regardless of location and/or number of light receiving pixels to be read out in the two signal readout modes, a stable OB clamping is effected even when signal readout mode has been changed or when location and/or number of pixels to be read out in each signal readout mode have been changed. The above main object is thereby achieved. Here, in this aspect, if FPN suppression means is provided, FPN suppression can be effected at a high accuracy.
Also, in this aspect, by setting the number of pixels to be read out in the mode for reading out one light receiving pixel out of every given number of pixels as the same as that in the mode of reading out the light receiving pixels in a given region, signal accumulating time is the same in the two modes so that stable images can be obtained as having an equivalent brightness before and after switching between the two modes.
It is another object of the present invention to provide an image pickup apparatus in which stable OB clamping is effected even when signal readout mode has been changed or when location and/or number of pixels to be readout have been changed and stable OB clamping is effected also in readout of all the pixels.
In accordance with the present invention, there is provided an image pickup apparatus including: an XY addressing type solid-state image pickup device having light receiving pixels and optically shielded OB pixels on the periphery of the light receiving pixels; and drive means for driving the solid-state image pickup device, in addition to a mode for reading out all the pixels, in signal readout modes of both or one of a thinned-out readout mode for reading out one light receiving pixel out of every given number of pixels and a mode of reading out the light receiving pixels in a given region; and the apparatus further including means for effecting OB clamping by always reading out OB pixels at the same location and holding an OB clamp reference value at the time of such OB clamping in the mode for reading out one pixel out of every given number of pixels and in the mode of reading out a given region regardless of these two signal readout modes or regardless of location and/or number of light receiving pixels to be read out in the two signal readout modes, and effecting OB clamping in the mode for reading out all the pixels by using the OB clamp reference value having been held in the mode for reading out one pixel out of every given number of pixels and/or the mode of reading out a given region.
By constructing in this manner, stable OB clamping is effected even when signal readout mode has been changed or when location and/or number of light receiving pixels to be read out in each signal readout mode have been changed, and at the same time it becomes possible to effect a stable and secure OB clamping with a reduced time lag even in the mode for reading out all the pixels such as at the time of taking one complete shot of picture, thereby achieving the above object.
Also, in this aspect, by setting the number of pixels to be read out in the mode for reading out one light receiving pixel out of every given number of pixels as the same as that in the mode of reading out the light receiving pixels in a given region, signal accumulating time is the same in the two modes so that stable images can be obtained as having an equivalent brightness before and after switching between the two modes.